Radio frequency (RF) heating is used for a wide range of industrial processing applications such as metal melting, welding, wood drying and food preparation. The output powers required range from a few kilowatts to values in the megawatt region. The frequency range can be a few hundreds of kilohertz to several tens of megahertz using triodes or tetrodes. For microwave applications of RF in the frequency range above 500 MHz it is usual, but not necessary, to use magnetrons.
Thermionic tubes require a heater supply to heat the thermionic cathode and in high power thermionic tubes the cathode is heated directly, i.e. the heater acts as the cathode. The use of the term “cathode”, “cathode heater” or “heater” throughout this document implies this definition where the context does not demand otherwise. With thoriated tungsten or pure tungsten cathodes used in such tubes the heater power required is usually quite high, for example 12V at 120 A implying a relatively low load resistance of 0.1 ohm. Also practical and convenient embodiments of the microwave generator frequently require that the heater circuit is operated not at ground potential but at an eht potential of 20 kV or higher.
Thus, in such embodiments, the cathode supply has to provide several kW of power to a low resistance load with a voltage isolation >20 kV. It is well-known to provide this power with a large power frequency transformer operating at 50 Hz or 60 Hz and constructed with large spacing and typically immersed in oil to provide high voltage isolation. Generally the voltage applied to the cathode has to be carefully controlled and adjusted during operation and thyristor regulators are used for this function, typically operating on the primary of a mains transformer.
It is important that the cathode, being one of the most fragile components of a magnetron, operates at its design temperature to prolong the life of the cathode by avoiding overheating while maintaining the required emissivity and preventing arcing by avoiding under heating. It is known in the art to seek to monitor the cathode temperature with a pyrometer, but with use of the magnetron the pyrometer window becomes occluded leading to false temperature readings. Alternatively, a varying schedule of power supplied, developed on a trial and error basis, may be applied during warm-up and operation of the magnetron.
Moreover, known transformers for supplying the heater current are expensive and very large, occupying a volume of 0.07 m3 and weighing 100 kg in the example given above. Moreover, thyristor controllers for power regulation are problematic in that they have limited control capabilities and poor transient response characteristics.
It is an object of the present invention at least to ameliorate the aforesaid disadvantages in the prior art.